For the upcoming National Geographic special Gender Revolution: A Journey With Katie Couric, the veteran journalist crisscrossed the nation to talk with scientists, psychologists, activists, authors, and families to better understand the complexities of gender.

As Couric says in the documentary’s affecting trailer, which debuts exclusively on EW, “It used to be so simple: You were or a boy or you were a girl. But that was then, and this is now.”

Timed to the gender issue of National Geographic Magazine, the two-hour special explores the fluidity of gender identity through scientific and social lenses, including the roles played by genetics, brain chemistry, and modern culture. The trailer glimpses Couric discussing what it’s like raising a transgender child, how pronouns inform our notions of self, and how the fight for transgender rights has reached Supreme Court.

It turns out, Couric says, “We’re all a lot more complicated than we’ve assumed.”